Electrical band-pass filters are used in many different types of consumer and industrial electronic product to select or reject electrical signals in a range of frequencies. In recent years, the physical size of such products has tended to decrease significantly while the circuit complexity of the products has tended to increase. U.S. patent application Ser. No. 10/699,298 of John D. Larson III entitled Stacked Bulk Acoustic Resonator Band-Pass Filter with Controllable Pass Bandwidth, of which this application is a continuation-in-part, discloses a highly miniaturized, high-performance, low-cost band-pass filter based on a decoupled stacked bulk acoustic resonator (DSBAR). A DSBAR is composed of stacked film bulk acoustic resonators (FBARs) and an acoustic decoupler located between the FBARs.
Transformers are used in many types of electronic device to perform such functions as transforming impedances, linking single-ended circuitry with balanced circuitry or vice versa and providing electrical isolation. However, not all transformers have all of these properties. For example, an auto-transformer does not provide electrical isolation. U.S. patent application Ser. No. 10/699,481 of John D. Larson III and Richard Ruby entitled Thin-Film Acoustically-Coupled Transformer, of which this disclosure is a continuation-in-part, discloses a highly miniaturized, high-performance, low-cost transformer that has one or more DSBARs each incorporating an acoustic decoupler. This film acoustically-coupled transformer (FACT) is capable of providing one or more of the following attributes at electrical frequencies in the range from UHF to microwave: impedance transformation, coupling between balanced and unbalanced circuits and electrical isolation. A FACT typically additionally has a low insertion loss, a bandwidth sufficient to accommodate the frequency range of cellular telephone RF signals, for example, a size smaller than transformers currently used in cellular telephones and a low manufacturing cost.
The above-described band-pass filter and FACT and other devices that incorporate one or more DSBARs, each incorporating an acoustic decoupler located between its constituent FBARs, will be referred to in this disclosure as decoupled stacked film bulk resonator devices or, more concisely, as DSBAR devices.
As disclosed in the above-mentioned U.S. patent applications Ser. Nos. 10/669,289 and 10/669,481 (the parent applications), DSBAR devices have a band pass characteristic having a pass bandwidth determined by the properties of the acoustic decoupler. In embodiments of the DSBAR devices disclosed the parent applications, the acoustic decoupler was embodied as a single acoustic decoupling layer. The acoustic decoupling layer of each DSBAR is a layer of an acoustic decoupling material having an acoustic impedance different from the acoustic impedances of the materials of the FBARs that constitute the DSBAR. The acoustic impedance of an acoustic decoupling material is the ratio of stress to particle velocity in the material and is measured in Rayleighs, abbreviated as rayl.
In practical embodiments, the acoustic decoupling material was a plastic material having an acoustic impedance less than the acoustic impedances of the materials of the FBARs. A typical plastic acoustic decoupling material has an acoustic impedance of less than ten whereas the materials of the FBARs have acoustic impedances of greater than 30. The pass bandwidth of such embodiments depends on the acoustic impedance of the acoustic decoupling material. Accordingly, the pass bandwidth of an DSBAR device would appear to be definable simply by selecting an acoustic decoupling material having the appropriate acoustic impedance.
In practice, it has proven difficult to define the pass bandwidth of an DSBAR device simply by choosing an appropriate acoustic decoupling material. Materials that have acoustic impedances in the range that produces the most typically-used pass bandwidths with typical FBAR materials and that additionally have the ability to withstand the high temperatures and the etchants used in the processing performed after the acoustic decoupling layer has been formed are actually few in number.
What is needed, therefore, is an alternative way of defining the pass bandwidth of an DSBAR device.